Process for improving the tensile properties of polybenzimidazole fiber or yarn



United States Patent Office 3,541,199 Patented Nov. 17, 1970 US. Cl.264-290 13 Claims ABSTRACT OF THE DISCLOSURE Foamable polybenzimidazolefiber or yarn (strand material) which undergoes foaming or explodingduring hot drawing is subjected, prior to the hot drawing step, to anon-degrading heat treatment at a temperature in the range of from about50 C. below the hot drawing temperature up to the degradationtemperature of the polybenzimidazole fiber or yarn for a period of timeof at least about 1 second.

RELATED APPLICATIONS This application is a continuation-in-part ofapplication Ser. No. 520,657, filed Jan. 14, 1966 now abandoned, whichapplication is assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION The present invention relates to a processfor improving the tensile properties such as tenacity of foamablepolybenzimidazole fibers, filaments, yarns and the like, sometimeshereinafter referred to generally as strand material. More particularly,the present invention relates to a process for heat treating foamablepolybenzimidazole fiber prior to the fiber being hot drawn so as toeliminate or overcome the foaming or exploding problem which frequentlyoccurs thereby yielding a polybenzimidazole fiber having improvedtensile properties.

Hot drawing or stretching polymer fibers is a well known method forincreasing the fiber tensile properties, particularly tenacity. It hasbeen found, however, that when foamable polybenzimidazole fiber, ashereinafter defined, and particularly foamable aromaticpolybenzimidazole fiber is subjected to a hot drawing operation, thefiber will foam or explode during the operation resulting in a porousfiber product having relatively low tensile properties.

Heretofore, it Was believed that the foaming problem was due in largepart to the presence of water in the polybenzimidazole fiber, the waterhaving been picked up primarily during the water washing step used toremove residual spinning solvent from the fiber. However, it has sincebeen found that even when the polybenzimidazole fiber is dried to removeessentially all of the wash liquid such as water, the resulting fiberfrequently still foams during the hot drawing operation. Thus, dryingalone is not the solution. It appears that certain unknown volatilematerials that may be formed during the polybenzimidazole polymerizationreaction and which tenaciously adhere or are chemically bound to thefibers may be chiefly responsible for the foaming problem during hotdrawing.

Several methods have been suggested for overcoming the foaming problem.For example, the deleterious foaming and exploding can be reduced by hotdrawing at low speeds, that is, the speed of the supply roll, on theorder of 5 meters a minute. However, such drawing operations are tooslow to permit large scale economic production.

Similarly, the foaming and exploding can be reduced by hot drawing atlow draw ratios on the order of about 1.5 :1 or less, for example, asdetermined by the ratio of the surface speed of the takeup or stretchroll to the surface speed of the supply roll. However, such low drawratios are undesirable because a high degree of orientation cannotthereby be achieved, and fibers of low orientation have low tenacityvalues.

Accordingly, the primar object of the present invention is to provide aprocess for improving the tensile properties of foamablepolybenzimidazole fiber or yarn, that is, strand material. Anotherobject is to provide a process for hot drawing foamablepolybenzimidazole fiber without foaming. These and other objects will beapparent from the following description,

In accordance with the present invention, foamable polybenzimidazolefiber or yarn, i.e., strand material, is subjected to a heat treatmentprior to the hot drawing operation. The heat treatment is conducted soas not to degrade the polybenzimidazole strand material, that is, anon-degrading heat treatment, at a temperature in the range of fromabout 50 C. below the hot drawing temperature up to the degradationtemperature of the polybenzimidazole material for a period of time of atleast about one second. I

The term foamable polybenzimidazole fiber or yarn is meant to includeany and all polybenzimidazole fiber or yarn (strand material) whichundergoes foaming or exploding or both during hot drawing, that is,drawing at a temperature above about 400 C., particularly when the hotdrawing is at a draw ratio of above about 1.5 :1, and at a drawing speedin excess of about 5 to 10 meters per minute.

It is important to note that not all polybenzimidazole strand materialundergoes foaming during hot drawing, irrespective of the drawing speedsand ratios. However, there is no present method for determining whetheror not foaming will occur without actually hot drawing the material in asimple test run. Hence, if foaming does occur, the hot drawing processof the present invention may be employed.

DETAILED DESCRIPTION OF THE INVENTION The starting polymerPolybenzimidazoles are a known class of heterocyclic polymers whichconsist essentially of recurring units of the following Formulas I andII. Formula I is:

wherein R is a tetravalent aromatic nucleus, with the nitrogen atomsforming the benzimidazole rings being paired upon adjacent carbon atoms,i.e., ortho carbon atoms, of

the aromatic nucleus, and R is a member of the class consisting of analiphatic (alkylene) group, a cycloaliphatic ring, an aromatic ring anda heterocyclic ring such as pyridine, pyrazine, furan, quinoline,thiophene, and pyran. Formula II is:

sisting essentially of the recurring units of Formula II and of FormulaI wherein R is an aromatic ring or a heterocyclic ring.

As set forth in US. Pat. No. 3,174,947 and Re. Pat. 26,065, which areincorporated herein by reference, the aromatic polybenzimadazoles havingthe recurring units of Formula Il may be prepared by self-condensing atrifunctional aromatic compound containing only a single set of orthodisposed diamino substituents and an aromatic, preferably phenyl,carboxylate ester substituent. Exemplary of polymers of this type ispoly-2,5(6)-benzimidazole prepared by the auto-condensation ofphenyl-3,4-diaminobenzoate.

As also set forth in the above-mentioned patents, the aromaticpolybenzimidazoles having the recurring units of Formula I may beprepared by condensing an aromatic tetraamine compound containing a pairof orthodiamino substituents on the aromatic nucleus with a dicarboxylcompound selected from the class consisting of (a) the diphenyl ester ofan aromatic dicarboxylic acid, (b) the diphenyl ester of a heterocyclicdicarboxylic acid wherein the carboxyl groups are substituents uponcarbon in a ring compound selected from the class consisting ofpyridine, pyrazine, furan, quinoline, thiophene and pyran and (c) andanhydride of a aromatic dicarboxylic acid.

Examples of aromatic polybenzimidazoles which have the recurringstructure of Formula I and which may be formed into fibers or yarnsinclude:

where the double bonds of the ethylene groups are intact in the finalpolymer.

The preferred aromatic polybenzimidazole fiber or yarn is one preparedfrom poly-2,2-(m-phenylene)-5,5'-bibenzimidazole, the recurring unit ofwhich is:

Any polymerization process known to those skilled in the art may beemployed to prepare the polybenzimidazole which may then be spun intofiber and if it foams during hot drawing, subjected to the heattreatment of the present invention.

With respect to aromatic polybenzimidazoles, preferably equimolarquantities of the monomeric tetraamine and dicarboxyl compound areintroduced into a first stage melt polymerization reaction zone andheated therein at a temperature above about 200 C., preferably at least250 C., and more preferably from about 270 to 300 C. The reaction isconducted in a substantially oxygen-free atmosphere, i.e., below aboutp.p.m. oxygen and preferably below about 8 p.p.m. oxygen, until a foamedprepolymer is formed. Usually, the first stage reaction is continueduntil a prepolymer is formed having an inherent viscosity, expressed asdeciliters per gram, of at least 0.1, and preferably from about 0.13 to0.3, the inherent viscosity (I.V.) as used in the present specificationand claims being determined from a solution of 0.4 gram of the polymerin 100 ml. of 97 percent H 80 at C.

After the conclusion of the first stage reaction, which normally takesat least 0.5 hour and preferably 1 to 3 hours, the foamed prepolymer iscooled and then powdered or pulzerized in any convenient manner. Theresulting prepolymer powder is then introduced into a second stagepolymerization reaction zone wherein it is heated under substantiallyoxygen-free conditions, as described above, to yield a polybenzimidazolepolymer product, desirably having an I.V., as measured above, of atleast 0.6, e.g., 0.80 to 1.1 or more.

The temperature employed in the second stage is at least 250 0,preferably at least 325 C., and more preferably from about 350 to 425 C.The second stage reaction generally takes at least 0.5 hour, andpreferably from about 1 to 4 hours or more.

Preparation of the fiber As is well known, the polybenzimidazoles aregenerally formed into fibers or filaments by solution spinning, that is,by dry or wet spinning a solution of the polymer in an appropriatesolvent such as dimethylacetamide, dimethylformamide, dimethylsulfoxideor sulfuric acid (used only in wet spinning) through an opening ofpredetermined shape into an evaporative atmosphere for the solvent inwhich most of it is evaporated (dry) or into a coagulation bath (wet),resulting in the polymer having the desired shape.

The polymer solutions may be prepared, for example, by dissolvingsufficient polybenzimidazoles in the solvent to yield a final solutionsuitable for extrusion containing from about 10 to 45% by weight ofpolymer, based on the total weight of the solution, preferably fromabout 20 to 30% by weight.

One suitable means for dissolving the polymer in the solvent is bymixing the materials at a temperature above the atmospheric boilingpoint of the solvent, for example, 25 to C. above such boiling point,and at a pressure of 2 to 15 atmospheres for a period of 1 to 5 hours.

Preferably, the above polymer solutions, after suitable filtration toremove any undissolved portions, are dry spun. For example, thesolutions may be extruded through a spinneret into a conventional typedowndraft spinning column containing a circulating inert gas such asnitrogen, noble gases, combustion gases or super heated steam.Conveniently, the spinneret face is at a temperature of from about to170 C., the top of the column from about 120 to 220 C., the middle ofthe column from about to 250 C., and the bottom of the column from aboutto 320 C. After leaving the spinning column, the fibers are taken up,for example, at a speed in the range of from about 50 to 350 meters perminute. The resulting as-spun fibers are then preferably subjected to aslight steam drawing treatment at a draw ratio of from about 1.1:1 to1.5 :1. This pre-wash drawing treatment is to be distinguished from thepost-wash hot drawing process of the present invention. The pre-washdrawing is employed in order to prevent the fibers from relaxing andfalling off the bobbin during the subsequent washing step.

Desirably, the fibers are next washed so as to remove residual spinningsolvent, e.g., so that the washed fiber contains less than about 1% byweight solvent, based on the weight of the fiber, and preferably so asto obtain an essentially spinning solvent-free fiber. Typically, asimple water wash is employed, however, if desired other wash materialssuch as acetone, methanol, methyl ethyl ketone and similardimethylacetamide-miscible and volatile organic solvents may be used inplace of or in combination with the water. The washing operation may beconducted by collecting the polybenzimidazole fiber on perforated rollsor bobbins, immersing the rolls in the liquid wash bath and pressurewashing the fiber, for example, for about 2 to 48 hours or more.

The washed fibers are then dried in any suitable type of apparatus suchas an electric oven to remove at least the major portion of the washliquid, for example, at least 60% by weight of the wash liquid,preferably at least 90% by weight, and more preferably essentially allof the wash liquid. If substantially less than about 60% of the washliquid is removed prior to subjecting the fiber to hot drawing the fibermay undergo foaming merely due to the excessive wash liquid content. Aspreviously mentioned, however, even if essentially all of the washliquid is removed, this alone will not overcome the foaming problemwhich occurs during the hot drawing operation, clearly indicating thatthe solution to the foaming is not merely drying.

The drying operation is conveniently conducted at a temperature of fromabout 150 to 300 C. for about 2 to 100 hours or more. Preferably, thedrying temperature does not exceed about 250 C. as above thistemperature degradation of the fiber may occur, particularly if anextended heating period, i.e., several hours, is employed. Degradationis the loss of physical properties of a polymer fiber due to severing ofthe polymer chains due to heat or chemical reactions or both.

Heat treatment As previously mentioned, certain polybenzimidazole strandmaterial undergoes foaming or exploding if it is subjected to hotdrawing subsequent to the drying operation. The essence of the presentinvention is the elimination of this problem by the use of a particularheat treating step or operation.

The heat treatment is conducted by heating the foam able fiber or yarnat a temperature no lower than about 50 C. below the temperatureemployed in the hot drawing operation, preferably from about 50 C. belowthe hot draw temperature, that is, the maximum temperature used duringthe hot drawing operation, up to about the degradation temperature ofthe polybenzimidazole fiber. The .so-called degradation temperaturevaries for each polybenzimidazole, but may be determined by sim- 'pleexperimentation. While the heat treatment temperature can be higher thanthe hot draw temperature up to the degradation temperature, mostpreferably the heat treatment temperature is essentially the same as thehot draw temperature, i.e., maximum temperature employed in the hotdrawing operation.

The heat treatment is conducted for at least about one second, as belowthis time period the exposure is insufiicient to achieve the desiredresults of overcoming the hot drawing foaming problem. Preferably, theresidence time for the heat treatment is from about 5 to 50 seconds and,more preferably, from about 5 to 30 seconds. Typically, the higher thetemperature the lower the residence time that is necessary to achievethe non-foaming fiber structure. Above about 50 seconds residence time,the fiber will usually undergo degradation, particularly at the highertemperatures, for example, above the hot draw temperature.

The heat treatment may be conducted in any suitable type apparatus orzone either on a batch, continuous or semi-continuous basis, providedthat the individual fiber, filament, or yarn is in a substantiallyexposed condition. For example, it has been found that the strandmaterial cannot merely be wound in a typical overlapping fashion onto abobbin, placed into a furnace and heated. While the fiber near the top,that is furthest from the bobbin surface, will have been sufficientlytreated so as not to foam during hot drawing, the fiber underneath, nearthe bobbin will still undergo foaming.

The preferred method for heat treating the strand material iscontinuously, for example, by passing the fiber or filament over a hotsurface such as a hot shoe, pin or plate or through a radiantly heatedzone such as a muffie furnace. The strand material may also becontinuously heated by helically Wrapping it (without substantialoverlapping) around an internally heated rotating circular 7 surface.Employing continuous treatment assures adequate exposure of thematerial.

With respect to a suitable batch treatment, the fiber may be wrapped,without substantial overlapping, onto a spool or bobbin and placed intoany conventional type heating zone such as an oven. Obviously, such abatch treatment is not practical on a commercial basis where speed andvolume production are important factors.

The fiber or filament used in the present invention conveniently has adenier of from about 1 to 20 or more. Normally higher fiber deniersrequire slightly higher heat treatment residence times within theaforementioned ranges in order to achieve the desired non-foamingcondition. The optimum or preferred residence time for eachpolybenzimidazole fiber and denier may be easily ascertained by simpleexperimentation.

It is important to note that the invention is equally applicable to allforms of foamable polybenzimidazole strand material, that is, fibers,filaments, yarns or threads and the like. With multi-filament materials,however, continuous heat treatment is even more preferred in order toachieve adequate exposure of the individual fibers or filaments.

Hot drawing As a result of the aforementioned heat treatment, thepolybenzimidazole strand material may be hot drawn without any foamingoccurring, thereby enabling non-porous polybenzimidazole material to beobtained. In addition, because of the heat treatment, high drawingspeeds, that is, the speed of the supply roll, on the order of 20 to 50meters per minute or higher may be used, the limiting factor being theparticular design of the drawing apparatus and the elastic properties ofthe fiber or yarn, and not the foaming of the polybenzimidazole asheretofore. Further, the elimination of the foaming problem allowshigher draw ratios to be used if so desired.

The resulting heat treated polybenzimidazole fiber or yarn is subjectedto a hot drawing treatment in any convenient hot drawing apparatus orzone, for example, by passing the fiber over a heated surface such as ahot roll, shoe, pin or plate or by passing the fiber through a radiantlyheated zone such as a muffle furnace.

' Preferably, the strand material is passed immediately from the heattreatment zone into the hot drawing zone in order to minimize exposureof the heat treated material to the atmosphere as polybenzimidazolefiber or yarn combines readily with the moisture in the air. The strandmaterial may absorb as much as 18% by weight of water from the air.

The hot drawing is suitably conducted at a temperature of above about350 C. up to about the degradation temperature of the polybenzimidazolefiber or yarn, preferably from about 450 to 650 C., and more preferablyfrom about 500 to 550 C.

The pre-heated strand materials may be drawn at any desired draw ratiobelow that at which they break, typically from above about 1.5 :1 to 3.5:l and preferably from about 2.011 to 3.2:1. The hot drawing residencetime, i.e., the time during which the fiber or yarn is being heated, isusually from about 1 to 50 seconds, preferably 5 to 30 seconds, and morepreferably 10 to 20 seconds. Substantially longer residence times at theelevated drawing temperatures may cause fiber degradation.

The resulting hot drawnpolybenzimidazole fibers or yarns, i.e., strandmaterials, of the present invention are characterized by a high degreeof thermal stability and show great resistance to degradation by heat,hydrolytic media and oxidizing media. They may be used, for example, indeceleration chutes for aircraft, chutes for re-entry capsules, hightemperature dust collector bags and nonfiammable clothing and fabrics.

The invention is additionally illustrated in connection with thefollowing example.

7 EXAMPLE A foamable polybenzimidazole yarn, namely, poly-2,2-(m-phenylene)-5,5-bibenzimidazole, which may be prepared in anyconvenient manner known to those skilled in the art as heretoforedescribed was used in each of the following runs. I

In each run in Table I below the yarn was drawn over a hot shoe at 425C. The method of thermal pretreatment employed was to pass the yarnthrough a radiantly heated 8 on a continuous basis at essentially thesame temperature as employed in the hot drawing operation for a periodof time in the range of from about to 30 seconds, and thereaftersubjecting the resulting heat treated material to a hot drawingoperation at a temperature in the range of from about 450 to 650 C. fora period of time in the range of from about 5 to 30 seconds and a drawratio from above about 1.5:1 to 3.5:1 to yield a polybenzimidazolestrand material having improved tensile properties.

mutfie furnace. 0 5. The process of claim 4 wherein a draw ratio in theTABLE I Draw speed (meters/min.) I

Elongation, Tenacity,

Thermal pretreatment R R2 Draw ratio percent grams/denier 'IE /z (1)NOne Control-N0 Pretreatment or drawing 116 1. 9 30 (2) 12 seconds, 482C 50 60 1. 2:1 65 3. El 31 (3) None 2.011 10 4.4 18 (4) 12 seconds, 482C 100 2. 0:1 It) 4. 7 20 (5) None 50 100 2. 0:1 Porous and weak(learning occurred) R1 is the rate of the feed or supply roll while It:is the rate of the take-up 0r starch roll. b TE %-Index of fiberorganization wherein T is tenacity at break in grams per denier and 1selongation in percent extension from original length at break in tensiletest. An explanation of this test and its significance is given in theTextile Research Journal 36, No. 7, pp. 5934302, July 1966.

The Table I indicates the effect that thermal pretreatment followed byhot drawing have on improving the tensile strength of foamablepolybenzimidazole strand material and further indicate that suchproperties can be achieved while working at fiber or yarn supply speedsin the order of 50 meters per minute.

It is also interesting to note that the yarn of run (5) had beenpreviously oven dried but still foamed.

Similar results are obtained when a mono-filament polybenzimidazole isused in place of the yarn as well as when other polybenzimidazole fibersor yarns such as those specifically mentioned heretofore are used.

The principle, preferred embodiment, and mode of operation of thepresent invention have been described in the foregoing specification.However, it should be understood that the invention which is intended tobe protected herein, may be practiced otherwise than as describedwithout departing from the scope of the appended claims.

We claim:

1. A process for improving the tensile properties of foamablepolybenzimidazole strand material, which process comprises subjectingthe strand material, prior to hot drawing, to a non-degrading heattreatment at a temperature in the range of from about 50 C. below thehot drawing temperature up to the degradation temperature of the strandmaterial for a period of time of at least about one second up to about50 seconds, and thereafter subjecting the resulting heat treatedmaterial to a hot drawing operation conducted at a temperature in therange above about 400 C. up to the degradation temperature of the strandmaterial for a period of time in the range of about 1 to 50 seconds andat a draw ratio above about 1.5 :1 up to that at which the strandmaterial breaks to yield a polybenzimidazole strand material havingimproved tensile properties.

2. The process of claim 1 wherein the heat treatment and hot drawoperations are conducted at essentially the same temperature.

3. The process of claim 1 wherein the heat treatment is conducted on acontinuous basis for a period of time in the range of from about 5 to 50seconds and the hot draw operation is conducted at a temperature in therange of from about 450 to 650 C. at a draw ratio of from above about1.521 to 3.5:1.

4. A process for improving the tensile properties of ioamable aromaticpolybenzimidazole strand material, which process comprises subjectingthe strand material, prior to hot drawing, to a non-degrading heattreatment range of from about 2.0:1 to 32:1 is used in the hot drawingoperation and the hot drawing residence time is from about 10 to 20seconds.

6. The process of claim 1 wherein the strand material is a fiber.

7. The process of claim 1,wherein the strand material is a yarn havingfrom 10 to 1000 filaments each filament having a denier of from about 1to 20.

8. The process of claim 1 wherein the polybenzimidazole ispoly-2,2'-(m-phenylene)-5,5'-bibenzimidazole,

9. A process for improving the tensile properties of foamablepoly-2,2-(m phenylene)-5,5-bibenzimidazole yarn, which comprisessubjecting the yarn, prior to hot drawing, to a non-degrading heattreatment on a continuous basis at essentially the same temperature asthe hot draw temperature for a period of time of from about 5 to 30seconds, and thereafter subjecting the resulting heat treated materialto a hot drawing operation at a temperature in the range of from about500 to 550 C., at a draw ratio of from about 2.011 to 3.2:1 for a periodof time of from about 10 to 20 seconds to yield a yarn having improvedtensile properties.

10. The process of claim 2 wherein the heat treatment is conducted on acontinuous basis for a period of time in the range of from about 5 to 50seconds and the hot draw operation is conducted at a temperature in therange of from about 450 to 650 C. at a draw ratio of from above about1.5:1 to 3.521.

11. The process of claim 5 wherein the strand material is a fiber.

12. The process of claim 5 wherein the strand material is a yarn having10 to 1000 filaments each filament having a denier of from about 1 to20.

13. The process of claim 5 wherein the polybenzimidazole ispoly-2,2-(m-phenylene) -5,5'-bibenzimidazole.

References Cited UNITED STATES PATENTS 3,414,645 12/1968 Morgan 264-]843,415,782 12/1968 Irwin et al 264205 3,441,640 4/1969 Santangelo 264203JULIUS FROME, Primary Examiner H. MINTZ, Assistant Examiner US. Cl. X.R.260-47; 264-345

